1. Field of the Invention
This invention relates to a pneumatic tire and more particularly to an improvement of a run-flat tire which can largely extend a runnable distance of the flat tire or a running distance at a run-flat state while controlling an increase of a tire weight.
2. Description of Related Art
There have hitherto been proposed various run-flat tires capable of temporarily running during the puncture of the tire, and some tires among them are commercialized.
In order to extend the runnable distance during the puncture of the run-flat tire, it is general to suppress heat generation resulted from repetitive deformation accompanied with the rotation of the tire by controlling an expansion deformation of a sidewall portion outward in the widthwise direction of the tire and a bending deformation of a bead portion outward in the widthwise direction of the tire at a rim flange portion called as xe2x80x9ca falling-down deformation of a bead portionxe2x80x9d against a load applied to the flat tire. For the purpose of controlling each of the above deformations, it has widely been performed to thicken a gauge of a crescent-shaped reinforcing rubber arranged at the inside of the sidewall portion, to thicken a thickness of a bead filler arranged at an outer peripheral side of a bead core, to increase the number of carcass plies, or the like.
In the conventionally proposed run-flat tires, however, it is unavoidable to significantly increase the tire weight, and there are some cases where the tire weight increases to about 30%. Such an increase of the tire weight has problems such as degradation of fuel consumption, increase of rolling resistance, degradation of ride comfort to vehicle and degradation of the other various performances.
Although the quantity of heat generation is lowered by controlling the deformation of the tire, the increase of the volume in the members constituting the tire particularly increases the quantity of heat generation during the running under a high load fairly exceeding over the above lowered quantity of heat generation, which obstructs the extension of the running distance at the run-flat state and becomes a serious problem.
It is, therefore, an object of the invention to solve the afore-mentioned problems of the conventional technique and to provide a pneumatic tire capable of largely extending the running distance at the run-flat state under a high load by sufficiently enhancing the rigidity of the bead portion while advantageously controlling the increase of the tire weight.
According to the invention, there is the provision of in a pneumatic tire, particularly a pneumatic radial tire comprising a tread portion, a pair of sidewall portions each extending inwardly from each side of the tread portion in a radial direction, a pair of bead portions each arranged at an inside of the sidewall portion in the radial direction, a carcass toroidally extending between a pair of bead cores each embedded in the respective bead portion and wound around the bead core from an inside of the tire toward an outside thereof in the radial direction to form a turnup portion, a bead filler arranged between a main body portion of the carcass and the turnup portion thereof and adjacent to an outer peripheral surface of the bead core, and a reinforcing rubber arranged at the inside of the sidewall portion in the widthwise direction of the tire and having substantially a crescent shape at cross section thereof, an improvement wherein at least one cord reinforcing layer made from a cord(s) extending in an angle direction of approximately 90xc2x0 with respect to a radial line segment is arranged in a region ranging outward from a vicinity of the bead core in the radial direction and has a width in the radial direction of the tire corresponding to 10-50% of a section height of the tire.
The term xe2x80x9csection height of the tirexe2x80x9d used herein means a section height defined in JATMA YEAR BOOK and concretely means half the difference between the overall diameter and the nominal rim diameter when the tire is mounted onto an approved rim and inflated at the recommended air pressure under no load. And also, the term xe2x80x9cradial line segmentxe2x80x9d used herein means a plane including a rotating axis of the tire.
In the pneumatic tire according to the invention, the reinforcing rubber arranged at the inside of the sidewall portion and close to an inner face thereof mainly contributes to bear the weight of a vehicle body during the puncture of the tire to thereby control the expansion deformation of the sidewall portion in the widthwise direction, so that stress concentration into the sidewall portion and the carcass can advantageously be prevented during the puncture of the tire. And also, the cord reinforcing layer is arranged at least in the bead portion, wherein the increase of the weight is fairly less as compared with the case of increasing the gauge of the reinforcing rubber and the thickness of the bead filler, whereby the rigidity of the bead portion against the falling-down during the running at the run-flat state under the high load can sufficiently be increased to largely extend the running distance at the run-flat state.
Considering only the expansion deformation of the sidewall portion, particularly a zone of the sidewall portion located in the vicinity of a position of a maximum tire width, a force restraining the expansion deformation can effectively be enhanced by increasing the gauge of the reinforcing rubber or increasing the number of the carcass plies, while the falling-down deformation of the bead portion under a working condition of a relatively low load can be countered by increasing the thickness of the bead filler or the number of the carcass plies. In the latter case, however, the falling-down deformation of the bead portion can not effectively be restrained when a higher load is applied to the tire, and the concentration of such a deformation in the bead portion can not be dispersed, and hence there is caused a fear of breaking a zone of the bead portion located in the vicinity of a rim flange.
For this end, in the pneumatic tire according to the invention, the cord reinforcing layer is arranged at least in the bead portion, whereby each of the falling-down deformation and the expansion deformation in a zone ranging from the bead portion to the sidewall portion is effectively controlled and the concentration of such deformations is prevented, so that the run-flat durability is largely improved even under the higher load without substantially increasing the gauge of the reinforcing rubber and the thickness of the bead filler.
The cord reinforcing layer in such a tire functions to control the outward expansion deformation of the sidewall portion in the widthwise direction of the tire in the flat tire based on a high modulus of elasticity of the cord constituting the cord reinforcing layer and extending substantially in the circumferential direction of the tire, and to effectively restrain the outward falling-down of the carcass ply in the widthwise direction at least in the bead portion under a large cross angle between the cord in the cord reinforcing layer and the cord in the carcass ply, and to prevent separation between mutual cords in the carcass ply accompanied with the expansion deformation of the sidewall portion as mentioned above, and hence the weight of the vehicle body during the puncture of the tire can effectively be supported by the cord reinforcing layer.
In the invention, the cord constituting the cord reinforcing layer extends in an angle direction of approximately 90xc2x0 with respect to the radial line segment, so that the cord is substantially continuous in the circumferential direction and can advantageously bear tension so as not to elongate in the circumferential direction. If the cord angle with respect to the radial line segment largely comes out from 90xc2x0, the cord becomes discontinuous in the circumferential direction and has free ends at the inside and outside in the radial direction, so that the above tension can not be born and the effect expected in the cord reinforcing layer can not sufficiently be developed and further the adhesion to the free end of the cord is insufficient to cause a fear of creating separation failure.
The cord reinforcing layer according to the invention is formed by spirally winding a single cord or a bundle of plural cords arranged side by side. In such a spirally wound structure, a joint part of the cord can be removed from the cord reinforcing layer to bear a larger tension in the circumferential direction, and hence the falling-down deformation of the bead portion can more effectively be restrained. In particular, when the bundle of plural cords is spirally wound, the formation of the cord reinforcing layer can be more facilitated. And also, the cord reinforcing layer may be formed by using a plurality of ring-shaped cords having different diameters.
Furthermore, the width of the cord reinforcing layer in the radial direction of the tire is within a range of not less than 10% but not more than 50% of the section height of the tire. When the width is not less than 10%, the rigidity can effectively be enhanced at least in the bead portion, preferably in a region ranging from the bead portion to the sidewall portion, while when the width is not more than 50%, the merit through the increase of the rigidity as compared with the increase of the tire weight can be ensured.
In the pneumatic tire according to the invention, therefore, the expansion deformation of the sidewall portion and the falling-down deformation of the bead portion are advantageously controlled under the action of the cord reinforcing layer without increasing the thickness of the bead filler and the gauge of the reinforcing rubber, whereby the effective extension of the running distance of the flat tire even under a higher load can be realized and also the increase of the tire weight can be controlled as small as possible.
In a preferable embodiment of the invention, the cord constituting the cord reinforcing layer is an organic fiber cord or a steel cord and has a modulus of elasticity of not less than 3.2 GPa, and an end count of cords in the cord reinforcing layer per a width of 10 mm is 3-10 cords.
In case of using the organic fiber cord, the tire weight can effectively be decreased as compared with the case of using the steel cord.
When the modulus of elasticity of the cord is not less than 3.2 GPa, the rigidities of the bead portion and the sidewall portion can sufficiently be enhanced to sufficiently control the expansion deformation of the sidewall portion and the falling-down deformation of the bead portion in the running at the run-flat state even under a higher load after the puncture of the tire.
When the end count is not less than 3 cords per 10 mm, the rigidities of the bead portion and the sidewall portion can sufficiently be enhanced, while when it is not more than 10 cords per 10 mm, the cord diameter can be ensured within an applicable range without extremely thinning it to advantageously enhance the rigidity of each respective portion.